Currently, single-crystal Si substrates are widely used as substrates for semiconductors. However, since the substrates are not necessarily suitable for recent high withstand voltage and high frequency due to the characteristics thereof, substrates of single-crystal SiC or single-crystal GaN are beginning to be used although they are expensive. For example, by using a semiconductor element using silicon carbide (SiC) which is a semiconductor material having a wider forbidden band width than silicon (Si) to constitute a power conversion device such as an inverter or an AC/DC converter, reduction of power loss that cannot be reached by a semiconductor element using silicon has been realized. By using the semiconductor element made of SiC, in addition to further reducing the loss accompanying electric power conversion as compared with the conventional art, weight saving, miniaturization and high reliability of the device are promoted. Moreover, the single-crystal SiC substrates are also being studied as raw materials for nanocarbon thin films (including graphene) as next generation device materials.
To manufacture these single-crystal SiC substrates and single-crystal GaN substrates, normally, (1) the single-crystal SiC substrates are manufactured by SiC sublimation method in which seed crystals are grown while SiC of high-purity SiC powder is sublimated at a high temperature of 2,000° C. or more, and (2) the single-crystal GaN substrates are manufactured by a method for growing seed crystals of GaN in high temperature and high pressure ammonia or by further heteroepitaxially growing GaN on sapphire or single-crystal SiC substrates. However, since the manufacturing steps are complicated under extremely severe conditions, the substrate quality and yield are inevitably low, making them very expensive substrates and hampering practical use and widespread use.
Meanwhile, on these substrates, the thickness that actually exhibits the device function is 0.5 to 100 μm in both cases, and the remaining thickness portion is a portion mainly playing the role of only mechanical holding and protection function during handling of the substrates, that is, a handle member (substrate).
Thereupon, in recent years, a substrate, in which a relatively thin single-crystal SiC layer which can be handled is bonded to a polycrystalline SiC substrate by intervening with a ceramic such as SiO2, Al2O3, Zr2O3, Si3N4 or AlN or a metal such as Si, Ti, Ni, Cu, Au, Ag, Co, Zr, Mo or W has been studied. However, in a case where the former (ceramic) IS interposed to bond the single-crystal SiC layer and the polycrystalline SiC substrate, it is difficult to make electrodes at the time of manufacturing the device since the ceramic is an insulator. In a case of the latter (metal), it is not practical because metallic impurities are mixed in the device and the characteristics of the device tend to deteriorate.
To solve these drawbacks, various proposals have been made so far. For example, Patent Document 1 (JP 5051962) discloses a method for affixing a source substrate, in which ion implantation of hydrogen or the like is performed on a single-crystal SiC substrate having a silicon oxide thin film, to, at the silicon oxide surface, polycrystalline aluminum nitride (intermediate support, handle substrate) with silicon oxide laminated on the surface, transferring a single-crystal SiC thin film to polycrystalline aluminum nitride (intermediate carrier), thereafter depositing polycrystalline SiC and thereafter putting it in an HF bath to dissolve the silicon oxide surface and separate. However, since the bonded surface of the silicon oxide surface is usually coupled tightly and strongly, HF hardly permeates the entire surface of the silicon oxide surface, particularly the central portion. Thus, this method has drawbacks that the separation is not easy, it takes too much time, and the productivity is extremely poor. Moreover, when a large-diameter SiC composite substrate is manufactured by using this invention, a large warp occurs due to a difference in coefficients of thermal expansion between the polycrystalline SiC deposited layer and aluminum nitride (intermediate support), which is a problem.
Furthermore, Patent Document 2 (JP-A 2015-15401) discloses a method for laminating a single-crystal SiC layer on a polycrystalline SiC support substrate by thermal bonding by contacting a polycrystalline SiC support substrate surface and a single-crystal SiC surface after, for a substrate difficult to be planarized at the surface, reforming the polycrystalline SiC support substrate surface to be amorphous with high-speed atomic beams without forming an oxide film as well as reforming the single-crystal SiC surface to be amorphous. However, in this method, not only the peeling interface of the single-crystal SiC but also a part of the inside of the crystal is degenerated by the high-speed atomic beams so that the precious single-crystal SiC does not quite recover to good-quality single-crystal SiC even by subsequent heat treatment. Thus, this method has a drawback that a high-performance device or a good-quality SiC epitaxial film is difficult to be obtained when used for a device substrate, a template or the like.
In addition to these drawbacks, in order to affix single-crystal SiC and polycrystalline SiC of the support substrate with the above technique, the surface roughness of the affixing interface is indispensable to have a smoothness of 1 nm or less in arithmetic average surface roughness Ra. However, since SiC is said to be a difficult-to-cut material next to diamond, even if the single-crystal surface is reformed to be amorphous, it requires extremely a lot of time for subsequent smoothing processes such as grinding, polishing or chemical mechanical polishing (CMP) and a cost increase is inevitable. In addition, since the poly-crystal has a grain boundary, it is difficult to perform amorphization by high-speed atomic beams uniformly in the surface, and the affixing strength and occurrence of warps are problems, which are major obstacles to practical application.